Food is often placed in a transparent plastic container that includes a base with a large volume cavity that holds the food and with a cover that closes the cavity. Buyers want to be assured that, after the food has been placed in the container, as by a clerk wearing plastic gloves at the food store, that the container has not been opened. There is a possibility that a customer will secretly open the container enough to taste a bit of the food before closing it (and possibly leave his/her germs in the food), and potential buyers want to know whether this has happened. A device that could be installed by the manufacturer of the containers, which could be almost automatically activated by a clerk at the food store when the container was filled and initially closed, and which indicated to customers when a container has been opened, would be of value. Food containers are sold at low costs so any such device should be of low cost and be easily activated by a store clerk.